The IP Multimedia Core Network Subsystem (IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP), and that builds an open and flexible service environment, supports multimedia applications, and is able to provide rich multimedia services for users.
In the IMS service architecture, the control layer and the service layer are separated, and the control layer only provides the necessary functions such as triggering, routing, charging and so on to the service layer other then providing specific services.
The service triggering and control function in the control layer is implemented by the Call Session Control Function (CSCF). The call session control function is divided into three types of Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF) and Serving-CSCF (S-CSCF), wherein the S-CSCF takes the main responsibility, and the I-CSCF is optional.
The service layer is composed by a series of Application Servers (ASs) and that is able to provide specific service serving. The AS might be an individual entity or might also exist in the S-CSCF.
The control layer (S-CSCF) controls the service triggering according to the subscriber's subscription information, invokes the services in the ASs, and achieves the service function. The AS and the S-CSCF can be called as a Server Equipment (SE).
The IMS network also has the Application Layer Gateway (ALG, including the signaling processing entity and the media processing entity), which is the equipment implementing the interaction of control signaling and media among different networks. The media path between two session terminals is split by the ALG into two sections, wherein one section is an intra-network media path, that is, the media path between the ALG and the session terminal which is in the same network as the ALG, and the other section is an inter-network media path, that is, the media path between another session terminal and the ALG.
An end-to-end equipment in the session is called as a User Equipment (UE) which is responsible for interacting with the user, some UEs have multiple ways for accessing the network, including the way for accessing the network through the 3GPP Packet Switch (PS) domain, or through other non-3GPP PS domain, or even through the Circuit Switch (CS) domain, and so on.
If the CS network is configured with the enhanced Mobile Switch Center (eMSC), the eMSC provides the Session Initial Protocol (SIP) interface to interact with the IMS network, and then the interaction between the IMS network and the CS network is achieved by the eMSC.
For a UE with multiple access ways, if the UE can only use one access way at a certain moment, when it is executing a service, such as calling, in one access way, and when the UE moves to another place and it needs to change its used access way, the UE and the network can provide a certain way so that the service being executed by the UE is not interrupted, and such a capability is called as the single radio terminal voice call continuity, Single Radio Voice Call Continuity (SRVCC) for short.
FIG. 1A is a flow chart of a method for implementing the existing PS to CS SRVCC and describes the process about the UE-1 and UE-2 have established the IMS session, the session signaling passes through the ALG and the service continuity application server (SC AS) so as to establish the IMS media connection path; the media connection path passes through the ALG, and after the single radio voice call continuity happens to the UE-1, how the UE-1 and the network implement to let the UE-1 use the CS domain to establish a media connection and to maintain the original session continuity. The process comprises the following steps.
In step A101, the UE-1 accesses the PS network, the HSS or the HLR sends subscriber data including the REFER number information to a PS network control entity (such as the MME or the SGSN), and the UE-1 establishes a session with the UE-2 through the PS and IMS networks.
In step A102, the access network decides to let the UE-1 switch from the PS domain to the CS domain according to the signal intensity information in the measurement report of the UE-1, the PS network control entity (such as the MME or SGSN) sends a switch request to the eMSC, the eMSC prepares the CS resources for the UE-1, and afterwards, sends a switch response to the PS network control entity.
In step A103, after the eMSC receives the switch request, the eMSC sends a switch request to the ALG, such as sending an INVITE message which takes the number or identifier of the ALG as the target, or an Initial Address Message (IAM) which takes the number of the ALG as the target, wherein the number or identifier of the ALG might be pre-configured in the eMSC.
In step A104, the ALG receives the switch request, updates the media connection between the UE-1 and the ALG, such as sending a reINVITE message.
In step A105, the ALG sends the response message of the switch request to the eMSC, such as sending a “200 OK” message or an Answer Message (ANM).
In step A106, after receiving the response message in step A102, the PS network control entity indicates the UE-1 to switch from the PS domain to the CS domain.
After the switch, a CS media connection is established between the eMSC and the UE-1, and a new IMS Media Connection 3 is established between the eMSC and the ALG, the eMSC connects the CS Media connection with the newly established IMS media connection 3, and the ALG connects the newly established IMS media connection 3 with the original IMS media connection 2 so that the UE-1 can continue the call with the UE-2.
FIG. 1B is a flow chart of a method for implementing the existing CS to PS SRVCC and describes the process about the UE-1 and UE-2 have established the session through the CS and IMS networks, the session signaling passes the SC AS), and after the SRVCC happens to the UE-1, how the UE-1 and the network implement to let the UE-1 use the PS domain to establish a media connection and to maintain the original session continuity. The process comprises the following steps.
In step B101, the UE-1 accesses the CS network, the UE-1 establishes a session with the UE-2 through the CS network and the IMS network, and a CS media connection is established between the UE-1 and the CS network control entity, and the IMS media connection 1 is established between the CS network control entity and the UE-2.
In step B102, the access network decides to let the UE-1 switch from the CS domain to the PS domain according to the signal intensity information in the measurement report of the UE-1, the CS network control entity (such as the MSC Server) sends a switch request to the eMSC, and the eMSC sends a switch response to the CS network control entity.
Step B102 can be skipped if the CS network control entity implements the eMSC function.
In step B103, after the eMSC receives the switch request, the eMSC sends a REFER request to the ALG, such as sending a REFER message which takes the number or identifier of the ALG as the target, wherein the number or identifier of the ALG might be pre-configured in the eMSC.
In step B104, the ALG receives the REFER request, it sends a REFER request to the SC AS, such as sending an INVITE message including the media resource information of the ALG; after the SC AS receives the REFER request, it updates the remote end, such as sending a reINVITE message or an UPDATE message to the UE-2, after the SC AS receives the response message from the UE-2, it sends a REFER response to the ALG.
In step B105, the ALG sends the response message of the REFER request to the eMSC, such as sending a “200 OK” message.
In step B106, the CS network control entity receives a response message in step B102, and then instructs the UE-1 to switch from the CS domain to the PS domain.
In step B107, after the UE-1 switches to the PS domain, it initiates the REFER request to the SC AS, such as sending an INVITE message including the media resource information of the UE-1.
In step B108, the REFER request passes the ALG, and the ALG sends a response message including the media resource information of the ALG to the UE-1.
At this point, an IMS media connection 2 is established between the ALG and the UE-1, and a new IMS Media Connection 3 is established between the UE-2 and the ALG, the ALG connects the IMS Media connection 2 with the newly established IMS media connection 3 so that the UE-1 can continue the call with the UE-2.
The ALG might be an entity dynamically allocated in the session establishment process, and the problem about how the eMSC dynamically determines the ALG so as to send a REFER request to the appropriate ALG in the aforementioned existing single radio voice call continuity process is not considered.